Hydrocarbon cracking furnace and its operation



Dec. 15, 1959 L. w. POLLOCK 2,917,564

HYDROCARBON CRACKING FURNACE AND ITS OPERATION Filed Jan. 5, 1959 CHARGEIN INVENTOR. L.W. POLLOCK BYM United States Patent 0,

Lyle W. Pollock, Bartlesville, Okla., assignor to Phillips PetroleumCompany, a corporation of Delaware Application January 5, 1959, SerialNo. 784,936

11 Claims. (Cl. 260-683) This invention relates to apparatus and methodfor cracking light hydrocarbons. 7

An object of this invention is to provide a cracking furnace and amethod for its operation for cracking light hydrocarbons. Another objectof this invention is to provide a furnace and a method for its operationfor cracking such light hydrocarbons as propane for the production ofethylene and propylene. Another object of this invention is to providesuch a method and apparatus in which the furnace tubes are constructedand operated in such a manner that they experience a relatively long anduseful life. Yet another object of this invention is to provide such anapparatus and operation that a greater throughput than normal is chargedto the furnace with the production of desired materials while realizingnormal depth of conversion. Yet other objects and advantages will berealized upon reading the following description which, taken with theattached drawing, respectively describes and illustrates a preferredembodiment of my invention.

In the drawing, Figure 1 illustrates in a diagrammatic form a side view,partly in section, of a preferred embodiment of my invention. Figure 2is a sectional view of my furnace taken on the line 2 2 of Figure 1.

In cracking light hydrocarbons to produce olefins in tube crackingfurnaces, the limiting factor on the capacity of the furnace is usuallythe temperature of the metal tube. Cracked gas etlluent must usually bein the neighborhood of 1500 F. to obtain high conversions of the feed.The maximum skin temperature (outside tube surface temperature) forstainless steel tubes for reasonable tube life is about 1600 F. Thetemperature difference between the cracked gas temperature and thetubeoutside wall temper-ature (skin temperature) is proportional to the heatflux or flow through the tube wall. By the term heat flux is meant theamount of heat, for example, B.t.u., that passes through the tube wallper square foot of outside surface of the tube per hour. Also, the heatflux for a given heat input per unit of cracked gas at constant gasvelocity varies directly as the tube diameter. It is desirable to use aslarge a tube as possible.

In order to increase the capacity of a cracking furnace, itis proposedto use variable size tubes in the furnace. Large inside diameter tubesare used for the first part of the cracking'furnace and as the skintemperature approaches the above-mentioned smaller inside diameter tubesare used in parallel. The heat flux, i.e., rate of heat transfer, forthe smaller diameter tubes will be less than for larger diameter tubesand thus the tube wall temperature will be less than would beexperienced on larger inside diameter tubes. Forexample, if a 6-inch ID.(inside diameter) tube were used for the first part p of the furnace, asthe maximum skin temperature is reached for the 6-inch tube, the flow isdivided between two 4-inch I.D. tubes. Again, as the 4-inch tubesapproach maximum skin temperature, the flow is divided between'fourparallel 3-inch I.D. tubes. As the 3-inch tubes approach their maximumskin temperature,'flow is divided between, for example, eight parallelZ-inch I.D. tubes. Then, as maximum skin temperature is approached inthese latter tubes, the material in process is withdrawn from thefurnaces as product.

Referring now to the drawing, reference numeral 11 identifies a pipeheater having more or less conventional furnace walls 15, ceramic liner47, a roof 19 and a stack 21. In the lower portion of the stack there isillustrated a-preheater coil 25 for preheating of the charge stock priorto its passage into the furnace proper.

In this embodiment a first heating tube 29 is a stainless steel tube of6-inch inside diameter (I.D.) by 36 feet long. When this tube isconsidered to include one return bend, the entire tube and return bendhave a length of about 38.5 feet. Tubes 31 are arranged for transmissionof fluid in parallel. These tubes 31 are of 4-inch ID. and 36 feet long.Tubes 33 are 3-inch I.D. tubes 36 feet in length while tubes 35 are2-inch ID. and 36 feet long. Each of the tubes 31, 33 and 35, whenincluding one return bend, is between 37.5 and 36.5 feet in length. Amanifold 37 communicates one end of tube 29 with the adjacent ends oftubes 31. A manifold 39 communicates the other ends of tubes 31 with theadjacent ends of tubes 33. Another manifold 41 communicates the otherends of tubes 33 with the adjacent ends of tubes 35 while a manifold 43communicates the outlet ends of tubes 35 with an outlet pipe 45. Aceramic lining 47 is provided throughout the interior walls of thefurnace as desired. Burners 49 are positionedat such locations asdesired in the side walls of this furnace. On reference to Figure 2,which is a cross sectional view of Figure 1, taken on the line 22, it isnoted that the several banks of tubes are staggered with respect to oneanother. A transfer pipe 27 connects the outlet end of the preheatercoil 25 with the inlet end of the 6-inch diameter heating tube 29. Apipe 23 is for inlet of charge stock to the furnace. These tubes aresupported by supports 17. I

In the operation of this furnace, charge stock, such as a propane stock,is introduced into the furnace through pipe 23 and passes through thepreheater coil 25. The preheated stock is then transferred by way oftransfer. pipe 27 to the inlet end of the first and large diameter tube29. From the opposite end of this tube the heated stock passes asdivided streams through the two parallel tubes 31 which, as mentionedabove, have smaller diameters than tube 29. The further heated gasesfrom the outlet ends of tubes 31 are again divided and substantiallyequal volumes are passed through four parallel tubes 33 for furtherheating. The outlet ends of these tubes, as stated, are connected bymanifold 41 and this further heated stock passes in equal streams to,for example, eight final heating tubes 35 in parallel. This finallyheated stock is then collected in manifold 43 and is finally withdrawnthrough outlet pipe 45.

The particular operation of this furnace is as follows. In the first andlarge diameter tube 29 heating is such that at about the outlet end ofthe heating tube the outside. surface of the tube reaches a temperaturein the neighborhood of about 1600" F., that is, the maximum allowableskin temperature of the tube. The gas is then removed from tube 29 andthe stream is divided into two portions and these are passed through atleast two tubes of smaller diameters than tube 29. Since the inner crosssections of the two tubes 31 are about equal to the cross sectional areaof tube 29, the velocity of the heated gases passing through tubes 31 isabout the'same as the velocity of the gas passing through tube 29. Thus,by the time the gasin process reaches the outlet end of tubes 31, theskin temperature of the tubes has approached a maximum allowablelimit'of about 1600311 streams from tubes 31 are combine'd 'or arepassed through separate manifolds, if desired, and again divided intosubstantially equal portions and passed through still smaller diametertubes 33. The combined cross sectional area of the four tubes 33 isapproximately the same as the combined area of tubes 31 and also thearea of tube 29 so that the gases pass through tubes 33 at approximatelythe same velocity as they passed through tubes 31 and through tube, 29.Thus, by the time the gas reaches the outlet ends of the several tubes33, the skin temperature of the tubes has again reached approximatelythe maximum of about 1600 F. At this point the four streams of gas fromtubes 33 are combined in manifold 41 or are, if desired, passed throughseparate manifolds, and are again divided into a larger number ofstreams as, for example, eight streams, for passing through the eightstill smaller diameter tubes 35. The combined cross sectional area ofthese eight tubes is approximately the same as the cross sectional areaof the four tubes 33, of the two tubes 31, and of the single tube 29, sothat the velocity of the gases passing through tubes 35 is about thesame as the velocity of the gases passing through the other tubes of thefurnace. Also, by the time the gas passing through the eight tubes 35reaches the outlet end of the tubes, the skin temperature of these tubeshas again reached the maximum allowable limit of about 1600 F., and bythis time conversion of the gases is sufiicient and the gases arecollected in manifold 43 and withdrawn from the furnace through theoutlet pipe 45.

Prior art heating furnaces for such cracking operations involve the useof, for example, either a single bank or multiple banks of, for example,6-inch inside diameter tubes. These 6-inch inside diameter tubes arearranged for serial heating of the gases undergoing conversion. If fouror more of the 6-inch diameter tubes were installed in a furnace, theywould be connected in series.

I find that I can employ a first tube of the 6-inch variety while inplace of the second 6-inch tube I substitute two smaller diameter tubesin parallel, for example, two 4-inch I.D. tubes. For the third 6-inchtube I substitute four 3-inch I.D. tubes in parallel and for the fourth6-inch I.D. tube I substitute eight 2-inch I.D. tubes in par-allel. Byreplacing the prior art tubes by successively smaller diameter tubes, Iam able to increase the degree of conversion on passing a given chargestock through the furnace; or, if desired, I can heat to a given degreeof conversion a larger amount of charge stock than in theabove-mentioned prior art operation. Furthermore, by adjusting theoperation I can increase the degree of conversion on a single passthrough the furnace and, at the same time, increase the throughput orcapacity of the furnace.

The following examples illustrate the utility of my furnace and itsoperation. In Example I are given operating data and stream compositionsat tubes No. 1, No. 2, No. 3 and No. 4 in a furnace using four 6-inchI.D. tubes, as in the prior art. It is to be noted that the skintemperature of the outside surface of the heating tubes at their outletends is held at approximately 1600 F. (between 1589' to 1605 F.). In thelast horizontal line of the tabulation of Example I it is noted that thepercent conversion of propane to ethylene and propylene is given. Thispercent conversion is obtained by dividing the mols of propane per moleof feed remaining in the stream by the mols of propane per mole of feedin the original charge stock, subtracting from 1.00, and multiplying by100. In this example the charge stock consisted of 58 mol percentpropane and 42 mol percent water as steam. It is further noted that thepercent conversion at the outlet of the fourth and final 6-inch diametertube was 40.2 percent.

EXAMPLE I.FOUR 6-INCH LD. TUBES Feed 11,400 lbs. propane and 3,370 lbs.steam per hour=58 mol percent propane per 42 mol percent water as steam.

4 Tube outer surface (skin) temperature about 1600 F. Four tubes, 6-inchID. by 36 feet long (38.5 feet includes return bend). 50 p.s.i.a. 40.2percent conversion of propane-total feed 14,770 lbs.

per hour.

Composition of tube efliuent (mols/ mols original feed) Tube Tube TubeTube N0. 1 No. 2 No. 3 No.4

Skin Temperature 1, 598 1, 589 1, 605

Outlet Gas Temperature... 1, 395 1, 415 1, 430

Heat Flux-B.t.u./hr.lsq.it 20, 655 18, 319 18, 351

Component:

Percent Conversion 11.4 21. 9 30. 7 40. 2

In Example II, the feed stock to the heater was the same as in ExampleI, but in place of the four 6-inch LD. tubes I use as tube No. 1 asingle 6-inch I.D. tube, as tubes No. 2, I use two 4-inch I.D. tubes, astubes No. 3, I use four 3-inch I.D. tubes and as tubes No. 4, I useeight 2-inch I.D. tubes. In this example the throughput or amount offeed stock charged to the furnace is the same as in Example I. It is tobe noted that the percentage conversion was increased to 57.5 percent.

EXAMPLE II.-TUBES OF UNLIKE I.D.'s

Feed 11,400 lbs. propane and 3,370 lbs. steam per hour=58 mol percentpropane per 42 mol percent water as steam.

Tube outer surface (skin) temperature about 1600 P.

All tubes 36 feet long; plus return bend.

57.5 percent conversion of propane-same throughout as Example I(14,770/lbs./hr.).

Composition of tube effluent (mols/100 mols original feed) Tube TubeTube Tube Unit Unit Unit Unit No. 1 No. 2 N0. 3 No. 4

No. of Tubes 1 2 4 8 ID. of Tubes, 1n. 6 4 3 2 Skin Temperature F" 1,604 1, 587 1, 588 1, 589 Outlet Gas Temperature F.. 1,377 1,430 1, 4701, 520 Heat FluXB.t.u./hr./sq. it 22, 676 21, 920 16, 076 13, 598

Component:

Percent Conversion 11. 4 23. 1 39. 2 57. 5

In Example III are given data for cracking the s me charge stock as inExamples I and H, but the feed rate was considerably higher than in thetwo preceding examples. The feed rate in this third example was 23,632pounds of combined feed per hour in contrast to 14,770 pounds ofcombined feed in Examples I and II. This third example is obtained fromsuch an operation as produced a degree or percentage of conversion of40.2 percent, that is, the same as in Example 1. Thus, it will beunderstood by those skilled in the art, when the feed rate is decreasedto a value less than 23,632 pounds combined feed per hour, the percentconversion is increased above the 40.2 percent of Example III. Thus, inoperation, one would decide on a feed rate between 14,770 pounds perhour and 23,632 pounds per hour to give a desired percentage ofconversion between 57.5% and 40.2%.

EXAMPLE IH.TU BES OF UNLIKE I.D. WITH GREATER THROUGHPUT Composition oftube efiluent (mols/100 mols original feed) Tube Tube Tube Tube UnitUnit Unit Unit No.1 No.2 No.3 No.4

No. of Tubes 1 2 4 8 Tube Size, I.D., 1n 6 4 3 2 Skin Temperature ofTubes F 1, 591 1, 595 l, 587 1, 584 Outlet Gas Temperature F 1, 381 1,435 1, 478 1, 529 Heat F1uxB.t.n./hr./sq. ft 28 775 30, 357 20, 899 16,115

Component:

Total 104. 31 109. 19 115. 51 121. 76

Percent Conversion 8.1 17. 1 28. 8 40. 2

It is realized by those skilled in the art that the specific insidediameters and the number of the several tubes used in each of the banksof tube coils, as described hereinabove, may be varied and altered forany given problem at hand. For example, tubes 31 can, if desired, bethree tubes or even four tubes, but when the larger number of tubes isused, they are, of course, of smaller inside diameter than the two4-inch tubes 31 illustrated. Likewise, when a larger number of tubes areused as tubes 33 and as tubes 35, they are, of course, of smallerdiameter than those illustrated herein.

Furthermore, if desired, more than one series of tubes, as tubes 29, 31,33 and 35, as illustrated herein, can be arranged in a single furnace.

The example, as given herein, uses conventional stainless steel tubes,such as Schedule 40 tubes, for the cracking operation. It is realized bythose skilled in the art that other hydrocarbons than propane arecracked in furnaces provided with heating tubes of other metals thanstainless steel. Regardless of the particular composition of the tubes,the principles set forth in this application apply. When usingconventional steel for the tubes, the maximum permissible skintemperature will obviously be considerably less than about 1600 F. Forexample,

butane, pentane or other higher boiling hydrocarbons which crack atlower temperatures than the temperature for propane can possibly usefurnace tubes of other and less expensive compositions. However, whenethane is cracked for the production of ethylene, as high a temperatureas possible taking into consideration a reasonable tube life, is used.The use of this furnace of my invention and the herein disclosed methodof operation serves to increase markedly the length of life of thefurnace tubes. Furnace tubes, particularly those of alloy materials, arevery expensive and any mode of operation and particular furnace.construction which increases the length of tube life is veryworth-while. The term schedule as applied to furnace tubes indicates, ingeneral, maximum allowable working pressure of a tube.

The herein-disclosed specific tube sizes are given merely as examples ofthe principles upon which my invention is based. It is realized thatvarious alterations in furnace construction as regards tube size,positioning, and the like, and method of operation, may be practiced andyet remain within the intended spirit and scope of my invention.

While certain embodiments of the invention have been describedfor-illustrative purposes, the invention obviously is not limitedthereto.

I claim:

1. A furnace comprising, in combination, a combustion chamber havingenclosing walls, a fuel burner operatively positioned in one of saidwalls, an exit for combustion gases from said combustion chamber, aplurality of radiant heating tubes disposed operatively in saidcombustion chamber, an inlet communicating with one end of a firstheating tube of said plurality of tubes, a first manifold communicatingthe other end of said first heating tube with adjacent ends of a pair ofsecond heating tubes of said plurality of tubes, said second tubeshaving smaller inside diameters than the inside diameter of said firstheating tube, a second manifold communicating the other ends of saidheating tubes with adjacent ends of third heating tubes of saidplurality of heating tubes, said third heating tubes having smallerinside diameters than the inside diameters of said second heating tubes,a third manifold communicating the other ends of said third heatingtubes with adjacent ends of fourth heating tubes, said fourth heatingtubes having smaller inside diameters than the inside diameters of saidthird heating tubes, a furnace outlet, a fourth manifold communicatingthe other ends of said fourth heating tubes with said outlet, said thirdheating tubes comprising a pair and at least one additional heatingtube, and said fourth heating tubes comprising at least one more heatingtube than the number of third heating tubes. 3

2. A furnace comprising, in combination, a combustion chamber havingenclosing walls, a fuel burner operatively positioned in one of saidwalls, an exit for combustion gases from said combustion chamber, aplurality of radiant heating tubes disposed operatively in saidcombustion chamber, an inlet communicating with one end of a firstheating tube of said plurality of tubes, a first manifold communicatingthe other end of said first heating tube with adjacent ends of at leasta pair. of second heating tubes of said plurality of tubes, said secondtubes having smaller inside diameters than the inside diameter of saidfirst heating tube, a second manifold communicating the other ends ofsaid second heating tubes with adjacent ends of third heating tubes ofsaid plurality of heating tubes, said third heating tubes having smallerinside diameters than the inside diameters of said second heating tubes,a third manifold communicating the other ends of said third heatingtubes with adjacent ends of fourth heating tubes, said fourth heatingtubes having smaller inside diameters than the inside diameters of saidthird heating tubes, a furnace outlet, a fourth manifold communicatingthe other ends of said fourth heating tubes with said outlet, said thirdheating tubes comprising at 17 least one additional heating tube morethan the number of second heating tubes, and said fourth heating tubescomprising at least one more heating tube than the number of thirdheating tubes.

3. The furnace of claim 1 wherein the longitudinal axes of the tubes ofsaid plurality of tubes are mutually parallel and said fuel burner isdisposed in a wall parallel to the longitudinal axes of said heatingtubes.

4. The furnace of claim 1 wherein said first heating tube is positionedhighest in said combustion chamber, and said second, third and fourthheating tubes being positioned successively below said first heatingtube in said combustion chamber.

5. The furnace of claim 1 wherein first, second, third and fourthheating tubes of said plurality of tubes have inside diameters of 6inches, 4 inches, 3 inches and 2 inches, respectively,

6. A method for converting in a tube heater a vaporous hydrocarbonconversion stock at amaximum conversion temperature consistent with longoperating life of the heater tubes, comprising passing said vaporoushydrocarbon stock through a first heating tube, passing hydrocarbon fromsaid first heating tube through a plurality of second heating tubes inparallel, the linear velocity of the hydrocarbon stock in said secondtubes being approximately the same as the linear velocity through saidfirst heating tube, maintaining the skin temperature of said heatertubes at approximately the maximum allowable skin temperature thereofand withdrawing converted hydrocarbon product from said second heatingtubes.

7. A method for converting in a tube heater a vaporous propane stock toethylene and propylene at a maximum conversion temperature consistentwith long operating life of the heater tubes, comprising passing saidvaporous propane stock through a first heating tube, passing heatedpropane stock from said first heating through a plurality of secondheating tubes in parallel, the linear velocity of the stock in saidsecond tubes being approximately the same as the linear velocity throughsaid first heating tube, maintaining the skin temperature of said heatertubes at approximately the maximum allowable skin temperature thereofand withdrawing converted product comprising ethylene and propylene fromsaid second heating tubes.

8. A method for converting in a stainless steel heating tube heater avaporous propane stock to ethylene and propylene at a maximum conversiontemperature consistent with long operating life of the heater tubes,comprising passing said vaporous propane stock through a first stainlesssteel heating tube, passing said propane stock from said first tubethrough a plurality of second stainless steel heating tubes in parallel,heating said first tube and said plurality of tubes in such a' mannerthat the maximum skin temperature consistent with long tube operatinglife is reached only near the outlet of said first tube and near theoutlets of said second tubes, and removing the heated stock from saidsecond tubes as the product of the operation.

9. The operation of claim 7 wherein said maximum skin temperature isapproximately 1600 F., and the velocity of said stock passing througheach second tube is approximately the same as the velocity of the stockpassing through said first tube.

10. A furnace comprising, in combination, a combustion chamber havingenclosing walls, a fuel burner operatively positioned in one of saidwalls, an exit for combustion gases from said combustion chamber, aplurality of radiant heating tubes disposed operatively in saidcombustion chamber, an inlet communicating with one end of a firstheating tube of said plurality of tubes, a first manifold communicatingthe other end of said first heating tube with adjacent ends of a pair ofsecond heating tubes of said plurality of tubes, said second tubeshaving smaller inside diameters than the inside diameter of said firsttube in such a manner that the linear velocity of fluid in said secondheating tubes is approximately the same as its linear velocity in saidfirst heating tube, a furnace outlet, and a second manifoldcommunicating the other ends of said second heating tubes with saidoutlet.

11. A furnace comprising, in combination, a combustion chamber havingenclosing walls, a fuel burner operatively positioned in one of saidwalls, an exit for combustion gases from said combustion chamber, aplurality of radiant heating tubes disposed operatively in saidcombustion chamber, an inlet communicating with one end of a firstheating tube of said plurality of tubes, a first manifold communicatingthe other end of said first heating tube with adjacent ends of a pair ofsecond heating tubes of said plurality of tubes, said second tubeshaving smaller inside diameters than the inside diameter of said firsttube in such a manner that the linear velocity of fluid in said secondheating tubes is approximately the same as its linear velocity in saidfirst heating tube, and outlet means for said second tubes.

References Cited in the file of this patent UNITED STATES PATENTS389,567 Hall Sept. 18, 1888 477,153 Pielsticker June 14, 1892 2,029,293Alther Feb. 4, 1936 2,216,471 Frame et al. Oct. 1. 1940 2,580,002Carn'er Dec. 25, 1951 UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No, 2 917 5e4 December 15 1959 Lyle Wa Pollock It ishereby certified that error appears in the printed specification of theabove numbered patent requiring correction and that the said LettersPatent should read as corrected below.

Column 7 line 36 after "heating" inserttube me Signed and sealed this23rd day of August 1960,

(SEAL) Attest:

KARL He AXLINE ROBERT C. WATSON Attesting Officer Commissioner ofPatents

1. A FURNACE COMPRISING, IN COMBINATION, A COMBUSTION CHAMBER HAVINGENCLOSING WALLS, A FUEL BURNER OPERATIVELY POSITIONED IN ONE OF SAIDWALLS, AN EXIT FOR COMBUSTION GASES FROM SAID COMBUSTION CHAMBER, APLURALITY OF RADIANT HEATING TUBES DISPOSED OPERATIVELY IN SAIDCOMBUSTION CHAMBER, AN INLET COMMUNICATING WITH ONE END OF A FIRSTHEATING TUBE OF SAID PLURALITY OF TUBES, A FIRST MANIFOLD COMMUNICATINGTHE OTHER END OF SAID FIRST HEATING TUBE WITH ADJACENT ENDS OF A PAIR OFSECOND HEATING TUBES OF SAID PLURALITY OF TUBES, SAID SECOND TUBESHAVING SMALLER INSIDE DIAMETERS THAN THE INSIDE DIAMETER OF SAID FIRSTHEATING TUBE, A SECOND MANIFOLD COMMUNICATING THE OTHER ENDS OF SAIDHEATING TUBES WITH ADJACENT ENDS OF THIRD HEATING TUBES OF SAIDPLURALITY OF HEATING TUBES, SAID THIRD HEATING TUBES HAVING SMALLERINSIDE DIAMETERS THAN THE INSIDE DIAMETERS OF SAID SECOND HEATING TUBES,A THIRD MANIFOLD COMMUNICATING THE OTHER ENDS OF SAID THIRD HEATINGTUBES WITH ADJACENT ENDS OF FOURTH HEATING TUBES, SAID FOURTH HEATINGTUBES HAVING SMALLER INSIDE DIAMETERS THAN THE INSIDE DIAMETERS OF SAIDTHIRD HEATING TUBES, A FURNACE OUTLET, A FOURTH MANIFOLD COMMUNICATINGTHE OTHER ENDS OF SAID FOURTH HEATING TUBES WITH SAID OUTLET, SAID THIRDHEATING TUBES COMPRISING A PAIR AND AT LEAST ONE ADDITIONAL HEATINGTUBE, AND SAID FOURTH HEATING TUBES COMPRISING AT LEAST ONE MORE HEATINGTUBE THAN THE NUMBER OF THIRD HEATING TUBES.
 6. A METHOD FOR CONVERTINGIN A TUBE HEATER A VAPOROUS HYDROCARBON CONVERSION STOCK AT A MAXIMUMCONVERSION TEMPERATURE CONSISTENT WITH LONG OPERATING LIFE OF THE HEATERTUBES, COMPRISING PASSING SAID VAPOROUS HYDROCARBON STOCK THROUGH AFIRST HEATING TUBE, PASSING HYDROCARBON FROM SAID FIRST HEATING TUBETHROUGH A PLURALITY OF SECOND HEATING TUBES IN PARALLEL, THE LINEARVELOCITY OF THE HYDROCARBON STOCK IN SAID SECOND TUBES BEINGAPPROXIMATELY THE SAME AS THE LINEAR VELOCITY THROUGH SAID FIRST HEATINGTUBE, MAINTAINING THE SKIN TEMPERATURE OF SAID HEATER TUBES ATAPPROXIMATELY THE MAXIMUM ALLOWABLE SKIN TEMPERATURE THEREOF ANDWITHDRAWING CONVERTED HYDROCARBON PRODUCT FROM SAID SECOND HEATINGTUBES.